The present invention relates to a developing device for an image forming apparatus and having a hard first developing roller or first conveying means formed with fine magnetic N-S poles on the periphery thereof, and a soft second developing roller or second conveying means for conveying toner, or single component type developer, electrostatically transferred from the first roller to an image carrier.
Generally, in a copier, facsimile apparatus, laser printer or similar electrophotographic image forming apparatus, a developing device is operable with one of a single component type developer, or toner, and a two component type developer or toner and carrier mixture. A device using only toner is feasible for miniaturization and basically maintenance-free, compared to a device using a toner and carrier mixture. However, the problem with the device using toner is that it is difficult to charge the toner evenly to a desired polarity. Toner particles charged to a polarity opposite to the desired polarity smear the background of a toner image and thereby deteriorate the image. Various kinds of schemes have been proposed to obviate this problem while making the most of the advantages of this type of developer.
The developing device using the toner may have a soft developing roller as toner conveying means. However, the soft roller is apt to suffer from a creep (permanent compression set) and fail to contact a photoconductive element, or image carrier, and a blade evenly. This prevents the blade from forming a uniform thin toner layer on the roller. The soft developing roller may be replaced with a hard developing roller in order to eliminate the above occurrence. The soft roller has customarily been combined with an image carrier implemented as a photoconductive belt. Hence, the soft roller is not practical without resorting to a drive mechanism including a drive roller and gears. Further, because the belt becomes offset due to an uneven tension distribution thereof, an extra mechanism must be provided against the offset.
Moreover, the conventional device, whether the developing roller be soft or hard, cannot eliminate the toner charged to the opposite polarity and, therefore, the background contamination attributable thereto.
In the light of the above, there has been proposed a developing device having both a hard first developing roller and a soft second developing roller. The first roller, or first conveying means, is formed with magnetic poles on the periphery thereof and magnetically causes the toner to deposit thereon. The toner is electrostatically transferred from the first roller to the second roller or second conveying means. The second roller is rotated to convey the toner to a developing position where an image carrier is located. With the two rollers, the device is capable of preventing the toner of opposite polarity from arriving at the developing position.
Specifically, the toner is usually charged by friction when it is passes through between the first roller and the blade. To charge the toner evenly, it is necessary to limit the amount of toner deposition on the first roller for a unit area. Should more than the limited amount of toner be deposited on and conveyed by the first roller, there would increase the amount of uncharged particles, particles of short charge, and particles of opposite polarity. The above conventional device cannot prevent the toner of short charge from arriving at the developing position although it can intercept the uncharged toner and the toner of opposite polarity. When a latent image is developed by the toner of short charge, the resulting toner image lacks a desired image density or a desired density ratio. In addition, when the toner deposits on the latent image in more than a predetermined amount, it melts, when transferred to a paper and fixed by a fixing unit, and runs into the white background of the paper, thereby defacing the image.
Assume that the amount of toner for a unit area is limited on the first roller in order to charge the toner evenly while obviating the deterioration of an image. Then, the amount of toner which can be transferred to the latent image formed on the drum is also limited. Hence, it is likely that the image fails to have a sufficient density. To settle this situation, the second roller may be rotated at a peripheral speed two to three times as high as the peripheral speed of the drum. This will successfully increase the amount of toner to deposit on the second roller for a unit area. However, if the peripheral speed of the second roller is excessively higher than that of the drum, a scavenging force, acting on the toner reached the drum, is intensified and blurs the leading edge of the image or concentrates the toner at the trailing edge of the image. Further, it is likely that the adhesion or smash of the toner occurs due to frictional heat, or that the toner is charged by friction at the developing position.
As stated above, although the second roller may be rotated at a higher peripheral speed than the drum in order to implement the amount of toner on the drum great enough to achieve a maximum image density, the peripheral speed of the second roller should be confined in a certain range. Therefore, it has been customary to determine the amount of toner deposition on the drum and the amount of toner deposition on the first roller by suitably balancing them with each other.